© 2006 Plant Management Network.
First Report of Powdery Mildew of Eschscholzia californica (California Poppy) caused by Erysiphe cruciferarum in North America
Dean A. Glawe, Plant Pathologist, Department of Plant Pathology, Washington State University, and Professor, College of Forest Resources, Box 352100, University of Washington, Seattle 98195
Corresponding author: Dean A. Glawe. email@example.com
Glawe, D. A. 2006. First report of powdery mildew of Eschscholzia californica (California poppy) caused by Erysiphe cruciferarum in North America. Online. Plant Health Progress doi:10.1094/PHP-2006-1213-01-BR.
California poppy (Eschscholzia californica Cham., Papaveraceae) is an annual species grown widely in landscapes in the Pacific Northwest. Once established, populations become self-seeding and require little care. During an ongoing study of Erysiphales in the Pacific Northwest, a powdery mildew fungus was collected repeatedly on this species in Seattle, King Co., WA since 2001. The fungus was determined to be Erysiphe cruciferarum Opiz ex Junell, a species not reported previously on this host in North America. This report documents the occurrence of the disease and provides information on the morphology and identification of the causal agent.
The disease typically was observed during August to October. Signs (Fig. 1) included amphigenous white to grayish, superficial mycelia occurring on leaves as well as stems and capsules. Severe symptoms or detrimental effects on infected plants were not noted. Mycelia produced appressoria (Fig. 2) that usually were lobed or occasionally simple, and foot cells that were straight, cylindrical, and measured (27-)32-79(-88.5) × (6.5-)7.5-9.5(-10) µm. Conidiophores formed conidia singly (Fig. 3). Conidia (Fig. 4) were ovoid to cylindrical, hyaline, measured (37-)42.5-57(-64.5) × (12-)14.5-20.5(-22.5) µm, and upon germination formed a germ tube near the end of the conidium. The teleomorph has not been observed.
The fungus was determined to be E. cruciferarum on the basis of anamorph morphology and host. Features including superficial mycelia, appressoria, conidiophores, and conidia fit features described for E. cruciferarum by Braun (1). The range of sizes for conidiophore foot cells and conidia are somewhat larger than Braun (1) described, but they overlap the ranges he gave and the length/width ratio for conidia (described as greater than 2) fit the observed fungus. Furthermore, the rather variable appressoria (ranging from lobed to simple) fit those of E. cruciferarum (1). The only other two species of Erysiphales Braun (1) listed on Papaveraceae are Erysiphe orontii Castagne [currently designated Golovinomyces orontii (Castagne) V.P. Heluta] and Leveillula taurica (Lév.) Arnaud. Both occur in the Pacific Northwest (3) but are easily distinguished from the E. cruciferarum. Golovinomyces orontii produces chains of conidia, unlike the single conidia produced by E. cruciferarum. Leveillula taurica mycelium is predominantly internal, with conidiophores emerging through stomata, and the first-formed conidium in each series is lanceolate; E. cruciferarum has entirely superficial mycelium and all conidia, including the first-formed, are cylindrical (1).
Despite examining material collected during several years the teleomorph was not found. In this respect the life cycle of E. cruciferarum seems to resemble other species of Erysiphales in western Washington in which the teleomorph appears not to occur (2). Where and how this species overwinters remains to be determined. In addition to E. californica, Braun (1) listed a large number of species of Brassicaceae, Papaveraceae, Capparidaceae, and Resedaceae as hosts for this fungus and noted reports that it includes both host-specific and plurivorous strains. In California E. cruciferarum occurs on Brassica rapa subsp. rapa (4). Thus, it appears possible that additional hosts of this powdery mildew fungus will be found in the northwestern USA.
This locally common disease tends to become noticeable in late summer and early autumn. Furthermore, infected leaves, which appear grayish because of mycelia and conidia, are not dramatic enough to be noticed by most home owners. It seems likely that the rather mild effects of this disease are responsible for the fact that this fungus was not reported previously.
1. Braun, U. 1995. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Jena.
2. Glawe, D. A. 2003. First report of powdery mildew of Nandina domestica caused by Microsphaera berberidis (Erysiphe berberidis) in the Pacific Northwest. Online. Plant Health Progress doi:10.1094/PHP-2003-1023-01-HN.
3. Glawe, D. A., du Toit, L. J., and Pelter, G. Q. 2004. First report of powdery mildew on potato caused by Leveillula taurica in North America. Online. Plant Health Progress doi:10.1094/PHP-2004-1214-01-HN.
4. Koike, S. T., and Saenz, G. S. 1997. First report of powdery mildew, caused by Erysiphe cruciferarum, on Broccoli Raab in California. Plant Dis. 81:1093.